Inductor DCR Current Sensing With Temperature Compensation: An Accurate, Lossless Approach For POL Regulators

Focus:

In one popular version of inductor dc resistance (DCR) current sensing used by POL regulators, a passive filter network is connected in parallel with the inductor. By emulating the inductor admittance with a low-pass sense network, a proportional voltage image of the inductor current is derived. A flat frequency response is achieved when the inductor time constant matches that of the RC sense network. With this method, time-constant matching and DCR temperature dependence must be addressed to achieve accurate current sensing across variations in load and temperature, which is critical to maintaining accurate current limiting. This article explains the requirements for time-constant matching and DCR temperature compensation, looks at the conventional approach to temperature compensation (and the option of inductors with low temperature coefficient of resistance), and then describes an alternative approach that leverages capabilities within newer PWM controllers to perform remote temperature sensing and temperature compensation of the inductor DCR. The article then describes an implementation of a POL regulator using this current-sensing and temperature-compensation scheme. This design is based on TI’s LM27403 voltage-mode controller and uses a 2N3904 npn transistor for temperature sensing. The article concludes by presenting measurements of 1) the BJT voltage waveforms at different temperatures and 2) the overcurrent protection setpoint across the operating temperature range.

What you’ll learn:

• How to implement accurate, lossless current sensing in point-of-load regulators
• How to implement accurate overcurrent protection over temperature in point-of-load regulators

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Author & Publication:

Timothy Hegarty, Texas Instruments, Tucson, Ariz., How2Power Today, Nov 15 2013

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